1. Field of the Invention
The present invention relates to an improvement in a linear motion ball bearing that finds an extensive utility in not only general machine manufacturing industries but also any other high technological machine industries.
2. Description of the Prior Art
Such a kind of ball bearing as mentioned above is disclosed in Japanese Patent Application Disclosure No. 57956-1988, wherein referring to FIGS. 9 to 11 of said disclosure (corresponding to FIGS. 3 to 5 of the present application), the ball bearing consists of an elongated rod member 4' and a cylindrical outer sleeve member 1' for inserting said elongated rod member 4' therewithin, while on each of the peripheral surface of said rod member 4' and the innerwall surface of said outer sleeve 1' is formed such a double threaded screw groove track with a locas that has been obtained with a loop of strap in the form of an elongated ellipse wound around the peripheral surface of an elongated rod member 4' from one end to the other with the same pitch, so as to form an endless spiral ball circulating track. In the prior art bearing, a double thread screw track is formed on the elongated member 4' consisting of adjacent parallel spiral grooves each having the same cross section and depth, whereas on the innerwall surface of the outer sleeve member 1' is formed a double thread screw track consisting of adjacent parallel spiral grooves each with different cross-sectional depth, one of which provides a groove cross section slightly smaller but the other of which provides a groove slightly greater than a half of the cross section of the ball. Thus, by inserting a plurality of balls between the mating spiral grooves of the outer sleeve member 1' and the elongated rod member 4', the two members can effect a relative linear movement under rotational movement of said elongated rod member 4' within said outer sleeve member 1' due to the bearing action of said balls.
However, the prior art technique encountered a substantial technical difficulty in the formation of a double thread screw track on the innerwall surface of the outer sleeve member 1' as it must provide adjacent parallel spiral grooves with different depths, inevitably accompanying poor dimensional accuracy and defective surface working of the products.
As the result, sliding resistance of balls traveling through the non-load spiral ball returning groove of the bearing becomes greater, which in turn increases sliding resistance of the rod member 4' through the throughbore of the outer sleeve member 1'.
It is an object of the present invention therefore to eliminate the above-mentioned technical disadvantages of the prior art bearing, wherein by simplifying the working required for the formation of double thread screw groove tracks on the innerwall surface of the outer sleeve member and the peripheral surface of the rod-like insert member, yet with high dimensional accuracy and excellent smoothness in worked surface thereof, thereby insuring low cost manufacture of a high-performance ball bearing with the least sliding resistance in operation.